The Civil Aviation Authority of Vietnam revealed the discovery this morning, but said it was too dark to be certain the object was part of the missing plane.

More aircraft will be dispatched to investigate the site in the morning.

The new discovery comes just hours after it was revealed the plane turned back before vanishing, with not two but potentially four passengers onboard travelling under stolen passports.

The overnight developments raises the possibility of a 9/11 style mid-air hijack, yet a massive international sea search has failed to find any wreckage of the Boeing 777 jet.

The plane - flight MH370 which was carrying 239 passengers and crew - vanished after it lost contact with ground controllers somewhere between Malaysia and Vietnam after leaving Kuala Lumpur early Saturday morning for Beijing.

Vietnamese air force jets spotted two large oil slicks yesterday, but it was unclear if they were linked to the missing plane, and no debris was found nearby.

An armada of 40 ships, together with 22 aircraft, have launched a huge search and rescue operation to solve the mystery - which has echoes of cult TV series Lost.

At the time of its disappearance, the weather was fine, the plane was already cruising at 35,000ft and the pilots didn't send a distress signal - unusual circumstances for a modern jetliner operated by a professional airline to crash.

Another pilot who was flying ahead of the missing Malaysia Airlines plane has revealed he made contact with it minutes after he was asked to do so by Vietnamese air traffic control and heard "mumbling" coming from the cockpit.

He used his plane’s emergency frequency to try and establish its position after authorities failed to make contact.

The captain, who wants to remain anonymous, said: "We managed to establish contact with MH370 just after 1.30am and asked them if they have transferred into Vietnamese airspace.

“The voice on the other side could have been either Captain Zaharie or Fariq, but I was sure it was the co-pilot.

“There were a lot of interference... static... but I heard mumbling from the other end.

“That was the last time we heard from them, as we lost the connection."

Families of those missing have been told to 'expect the worse' [AFP]

“From what we have, there was no such distress signal or distress call per se, so we are equally puzzled”

Malaysia Airlines chief executive Ahmad Jauhari Yahya

Malaysia's air force chief Rodzali Daud didn't say which direction the plane might have taken or how long for when it apparently went off route.

"We are trying to make sense of this.

"The military radar indicated that the aircraft may have made a turn back and in some parts, this was corroborated by civilian radar."

Malaysia Airlines chief executive Ahmad Jauhari Yahya said pilots were supposed to inform the airline and traffic control authorities if the plane does a U-turn.

"From what we have, there was no such distress signal or distress call per se, so we are equally puzzled," he said.

Authorities were checking on the suspect identities of at least two passengers who appear to have boarded with stolen passports. On Saturday, the foreign ministries in Italy and Austria said the names of two citizens listed on the flight's manifest matched the names on two passports reported stolen in Thailand.

This, and the sudden disappearance of the plane that experts say is consistent with a possible onboard explosion, strengthened existing concerns about terrorism as a possible cause for the disappearance.

Al-Qaida militants have used similar tactics to try and disguise their identities.

But Clive Williams, a counter-terrorism expert at Australia's Macquarie University and a former military intelligence officer, said he doubted the two stolen passports aboard the flight were related to the disaster.

He said latest Interpol data showed there were 39 million lost or stolen passports reported as of December 13, 2013.

"Any flight of that size in Asia would be carrying a couple of people with false passports," he said. "When you think about the number of passports that have been stolen or gone missing around the world ... it could be related, but it's probably not."